The present invention relates to an electromechanical resonator fabricated by using the MEMS (Micro Electro Mechanical Systems) and a manufacturing method thereof.
In related electromechanical devices, configurations for operating an oscillator with various driving systems are applied. An electromechanical element that translates in parallel with respect to a substrate is used in a large number of devices. To mechanically operate the oscillator of such an electromechanical device, an input/output terminal of a fixed electrode arranged over the side face of the oscillator is required.
A method has been proposed for changing the vibration mode by changing the thickness of an electrode in the electromechanical devices.
For example, an electromechanical resonator described in Non-patent Reference 1 is proposed.
As shown in FIG. 18, the electromechanical resonator includes a beam-shaped oscillator which has both ends supported as free ends. A uniform air gap 53 is formed between a fixed electrode 51 and an oscillator 52 provided on a substrate 50. An electrostatic force is exerted in the air gap to control the oscillator to vibrate mechanically. While a resonance frequency is determined mainly by a vibration mode that depends on the electrode arrangement of the fixed electrode 51, a secondary flexural vibration mode is excited (FIG. 18(b)) in a configuration where two electrodes 51 of input and output terminals are arranged in the lower layer of the oscillator 52 as shown in FIG. 18(a). When three electrodes 51 are arranged in the lower layer of the oscillator 52, a tertiary flexural vibration mode is excited at a resonance frequency higher than that of the secondary mode. Positioning of the electrode arrangement means that the vibration mode is different depending on an arrangement of the electrode 51 with respect to an anti-node position of vibration.
Therefore, in case an electromechanical resonator having various resonance frequencies is required, it is necessary to make variable the size and arrangement of a fixed electrode depending on the vibration mode to be excited. Also, a manufacturing method for simply manufacturing such an electrode is required.
Patent Reference 1 describes a method for manufacturing an electronic gun and a quantum wire and uses a structure where a sacrifice layer and an electrode layer are formed in an upper layer of a triangular section structure. This manufacturing method causes the apex of an electrode to protrude and removes the protruding part of the electrode to form a final electrode by applying a resist of a thick film and through etching-back. The electrode is formed over each of the side faces of the structure and the electrodes serve as input and output terminals.
By using a manufacturing method for fabricating, as described in [Patent Reference 1], fixed electrodes to be arranged over the side faces of an oscillator in a single layer, is made possible by simultaneously forming multiple fixed electrodes in an electrode layer formed with one process step.
[Non-patent Reference 1] M. Demirci, C. T.-C. Nguyen, “Higher-mode Free-Free Beam Micromechanical Resonators,” Proceeding, 2003 IEEE Int. Frequency Control Symposium, Tampa, Fla., May 5-8, 2003, pp. 810-818.
[Patent Reference 1] JP-A-06-310029
A resonator of the vertical driving type described in Non-patent Reference 1 determines a resonance frequency of a selected vibration mode depending on the arrangement of fixed electrodes. In this configuration, the electrodes are formed only in a lower layer of an oscillator. Therefore, the electrodes are bounded in limited locations.
Patent Reference 1 uses a configuration where electrodes are arranged over the side faces of an oscillator and the electrodes are driven in parallel on a substrate. This approach solves the problem of Non-patent Reference 1. However, in Patent Reference 1, the height of an electrode arranged over each of the side faces of the structure is determined by the control time of resist etching-back in the manufacturing process, thus requiring sophisticated film thickness control in order to vary the electrode height. It is difficult to independently control the film thickness of multiple fixed electrodes. With this approach, it is necessary to have a multilayer electrode configuration. A multilayer configuration adds to the number of processes to complicate the manufacturing method with a rise in the manufacturing costs.
In case an electromechanical resonator of the parallel driving type is provided with the configuration of Patent Reference 1, it is necessary to remove the sacrifice layer in the lower layer of the oscillator to form released structure. FIG. 19 shows a cross section of this electromechanical resonator. A sacrifice layer 54 in the lower layer of an oscillator 56 is removed in FIG. 19. In case an electrode 55 is thin, the sacrifice layer 54 in the lower layer of the electrode 55 is simultaneously etched. As a result, undercutting may occur in the electrode. This results in problems that the electrode 55 is stuck and fixed to a substrate 57 and that the electrode 55 is excited with an input signal.